1. Field of the Disclosure
This disclosure generally relates to an input device and, more particularly, to a navigation device with an adjustable sample period.
2. Description of the Related Art
The optical mouse generally calculates the displacement according to the correlation between two image frames. However, high operation resources are required to perform the calculation on the whole image frame and thus it is possible to perform the calculation on only a part of the image frame.
For example referring to FIG. 1A, it is a schematic diagram of two consecutive image frames F1 and F2 captured by a camera. A 5×5 correlation search window SW, assuming at the center of the image frame F1, for calculating the displacement between the image frames F1 and F2 is computed by shifting the comparison image frame F2 around the reference image frame F1. For example, in calculating the element “13” of the correlation search window SW, the comparison image frame F2 is fully overlapped with the reference image frame F1 as shown in FIG. 1B such that 13×13 pixels of the two image frames F1 and F2 are all used to calculate the correlation. For example, in calculating the element “8” of the correlation search window SW, the comparison image frame F2 is overlapped with the reference image frame F1 by 1-pixel shifting upward as shown in FIG. 1C such that only 12×13 overlapped pixels of two the image frames F1 and F2 are used to calculate the correlation. For example, in calculating the element “1” of the correlation search window SW, the comparison image frame F2 is overlapped with the reference image frame F1 by 2-pixels shifting upward and leftward as shown in FIG. 1D such that only 11×11 overlapped pixels of the two image frames F1 and F2 are used to calculate the correlation. Other elements “2” to “7”, “9” to “12” and “14” to “25” in the correlation search window SW are calculated through similar processes.
Referring to FIG. 1A again, under the condition of limiting the displacement between two image frames within two pixel distances and assuming that the size of each pixel is 30 micrometers, an initial velocity (e.g. the velocity at time t1) is 5 inches/second and the acceleration requirement is 50 g, the sample period Δt is calculated to be smaller than 0.2994 ms by using the equation of uniformly accelerated motion, i.e. the frame rate is larger than 3,340 frames/second, if correct displacement is to be obtained, wherein “g” is the acceleration of gravity.
However, inventors notes that although the displacement is detected correctly by increasing the frame rate, the data quantity to be processed per second is also increased thereby degrading the power saving effect.